We have investigated the effects of thickness variation and thermal treatment of the electrode polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) in photovoltaic and photodetector devices using conjugated polymer blends as the photoactive material. By variation of the PEDOT:PSS layer thickness between 25 and 150 nm, we found optimum device performance, in particular low dark current and high external quantum efficiency (EQE) and open-circuit voltage (Voc), at around 70 nm. This has been observed for two different active layers. Annealing studies on the PEDOT:PSS films, with temperatures varied between 120 and 400 °C, showed an optimum device performance, in particular EQE and Voc at 250 °C. This optimum performance was found to be associated with loss of water from the PSS shell of the PEDOT:PSS grains. For annealing temperatures above 260 °C, device performance was dramatically reduced. This was associated with chemical decomposition leading to loss of sulfonic acid, although...